1. Field of the Invention
The present invention relates to a camera device for performing a shooting operation by converting, for example, image light from a subject into an electrical signal using an image pickup device. More particularly, the present invention relates a camera device which makes it possible to automatically perform a predetermined backlight correction operation by detecting a backlighted state as a result of determining the ratio between the brightness level of a portion of an imaging surface of the image pickup device having the lowest brightness level and the brightness level of a portion of the imaging surface of the image pickup device other than that of the portion having the lowest brightness level.
2. Description of the Related Art
Hitherto, a camera device which makes it possible to prevent the taking of underexposed photographs by performing an overexposing correcting operation when it is determined that a subject is in what is called a backlighted state has been known. The technology of such a camera device is disclosed, for example, in Japanese Unexamined Patent Publication No. 62-110369. In the invention which is disclosed in this document, the tendency of the main subject being positioned at the center of a screen with high probability is made use of. The imaging screen is divided into the center portion and the portions in the vicinity of the center portion. The brightness level of each portion is obtained in order to adjust the exposure by the ratio between that of the center portion and those of the portions in the vicinity, whereby the main subject at the center of the screen is suitably exposed.
More specifically, in the camera device, when the brightness of the subject at the center area of the imaging screen and the brightness of the subject at the vicinity of the center area are detected, and it is found that the brightness of the subject at the center area is less than the brightness of the subject at the vicinity of the center area by an amount equal to or greater than a predetermined value, a determination is made that the subject at the center area is in a backlighted state, so that an overexposing correcting operation is carried out during the shooting operation. When this is carried out, it is possible to prevent the mistake of taking a photograph in which the subject at the center area is squashed and appears black due to underexposure.
In addition, hitherto, a camera device for increasing an exposure standard value which is incorporated in the inside of the camera in order to perform an overexposing correcting operation when it is determined that the subject is backlighted is known. More specifically, in this camera device, a determination is made as to whether the level of an integration signal which is obtained when an output signal of, for example, an image pickup device (that is, a charge-coupled device (CCD)) passes through an optical detecting circuit (OPD) is smaller than or greater than the exposure standard value which is incorporated inside the camera. If it is greater, an underexposing correcting operation is carried out, whereas, if it is smaller, an overexposuring correcting operation is carried out. In the backlighted state, the exposure standard value is increased to perform the overexposing correcting operation. When this is done, it is possible to prevent the mistake of taking a photograph in which the subject at the center area is squashed and appears black due to underexposure.
Further, in recent years, a camera exposure control device has been disclosed in Japanese Unexamined Patent Publication No. 2-268080. In this device, in order to obtain a proper photometric value, a plurality of high-brightness clip circuits having different clip levels are provided. The device makes use of brightness signals which pass through the high-brightness clip circuits having clip levels in accordance with shooting scenes. According to this device, when a determination is made that the shooting scene is backlighted, the brightness signal of the subject is input through a high-brightness clip circuit having a low clip level in order to determine the photometric value. By adjusting the aperture of the iris diaphragm so that the photometric value is equal to the standard value, the mistake of taking a picture in which the subject at the center area is squashed and appears black due to underexposure is prevented from occurring.
In the method of performing an overexposing correcting operation by increasing the exposure standard value as described above, when, for example, a moving subject, such as an automobile, is to be photographed, the exposure standard value is increased and decreased many times under the environment in which determinations that the subject is backlighted and determinations that the subject is not backlighted are mixed. Therefore, it may be necessary to think of ways to prevent unstable exposure controlling operations, such as by performing a delaying operation.
On the other hand, in the method where the high-brightness component level of an integration signal (which is obtained after the output signal of the image pickup device (CCD) has passed through the optical detecting circuit (OPD)) is continuously previously measured in order to continuously change the high-brightness clip level in accordance with the degree of backlighting, the level of the integration signal of the optical detecting circuit (OPD) becomes smaller the more the clipping operation is performed. Accordingly, as the exposure controlling operation, an overexposing controlling operation is carried out as in the case where a determination that the amount of light incident upon the image pickup device (CCD) is small is made. Here, as shown in FIG. 11, it is not necessary to increase or decrease the exposure standard value, so that the conventional exposure controlling operation can be made use of as it is, making it possible to correspondingly maintain the reliability of the system.
However, in the method where the brightness of the subject at the center area and that of the subject in the vicinity thereof are compared as described above, when the main subject is located at both the center area and the vicinity of the center area of the screen, a difference between the brightness at the center area and that at the vicinity of the center area is decreased. Therefore, exposure correction may not be carried out because it sometimes cannot be determined that the subject is in a backlighted state even if it is in a backlighted state. In that case, the main subject is underexposed.
In addition, the main subject is not always at the center area of the screen. When it is at the vicinity of the center area and is backlighted, the brightness of the subject at the vicinity is lower than the brightness of the subject at the center area. Therefore, exposure correction may not be carried out because it sometimes cannot be determined that the subject is in a backlighted state even if it is in a backlighted state. In that case, the main subject is also underexposed.